Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, may have the capability to communicate data with what may be called an external programmer via a radio frequency telemetry link. A clinician may use an external programmer to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker may be modified after implantation of the pacemaker into the body in this manner. Modern implantable devices may also include the capability for bidirectional communication so that information may be transmitted to the external programmer from the implanted device for monitoring purposes. Data which may be transmitted from an implantable device to the external programmer may include various operating parameters and physiological data, either collected in real-time or stored from previous internal monitoring operations.
Telemetry systems for implantable medical devices utilize radio frequency energy to enable bidirectional communication between the implantable device and an external programmer. A radio frequency carrier may be modulated with either analog or digital information by amplitude shifting (i.e., AM [if analog] or ASK [if digital]), frequency shifting (i.e., FM [analog] or FSK [digital]), phase shifting (i.e., PM [analog] or PSK [digital]), or some combination of the above, such as Quadrature Amplitude Modulation (i.e., QAM). The external programmer transmits and receives the radio signal with an antenna.
In many previous systems the implanted antenna efficiency is very poor, and thus the antenna for the external device must be positioned in close proximity to the implanted device. In some previous systems, the implantable device and the external programmer communicate by generating and sensing a modulated electromagnetic field in the near-field region with the antennas of the respective devices inductively coupled together. The external antenna must therefore be close to the implantable device, typically within a few inches, in order for communications to take place. This requirement is inconvenient and limits the situations in which telemetry can take place.
While far-field RF telemetry has also been proposed, there exists a problem in the art of poor antenna efficiency. There also exists a problem with an implanted antenna having transmission field strengths that are not omni-directional (i.e., relatively similar strength in all directions) and the resulting antenna efficiency may depend upon the relative position of the body into which the device has been implanted. For example, an antenna that works well when the body is in a vertical position, may not have sufficient efficiency to operate when the body is in a horizontal position. The RF tuning circuits of some of the previous devices may also present a problem of maintaining adequate efficiency in the variety of possible device implant sites (different surrounding tissues have different dielectric constants), in the variety of different patient body types, and in a single patient over time, as the composition of the body into which it has been implanted changes with time, for example if a patient gains weight after the surgical insertion of a cardiac pacemaker.